1. Field of the Invention
The present invention relates to a paper feeding device used for feeding paper sheets in an image forming apparatus, such as a printer or a copying machine.
2. Description of the Related Art
In a printer, a copying machine or the like for recording an image in accordance with an electrophotographic process, the image is recorded in the following manner. Specifically, an electrostatic latent image corresponding to an image to be formed is formed on the surface of a photoreceptor. The electrostatic latent image is developed into a toner image. This toner image is transferred onto a paper sheet and is further fixed thereto. Consequently, image formation is achieved.
Such an image forming apparatus is provided with a paper feeding device for feeding paper sheets to the photoreceptor at a predetermined timing. The paper feeding device comprises, for example, a paper feeding cassette containing paper sheets, a paper feeding roller for taking a paper sheet from the paper feeding cassette, and a pair of registration rollers for stopping the paper sheet taken out by the paper feeding roller short of the photoreceptor and feeding the paper sheet to the photoreceptor a predetermined timing.
Such a paper feeding device generally has a paper detecting switch provided on a paper conveying path on the upstream side of the registration rollers with respect to the paper conveyance direction, that is, between the registration rollers and the paper feeding roller. This paper detecting switch is turned on when the paper sheet passes through the position where it is provided, to generate a predetermined signal. It is possible to judge that a paper jam has occurred between the registration rollers and the paper feeding roller on the basis of the output of the paper detecting switch. Specifically, when the paper detecting switch is held in an ON state over a time longer than the time required for the paper sheet to pass through the paper detecting switch after the registration rollers are driven, it is judged that a paper jam has occurred. When it is judged that a paper jam has occurred, the image forming apparatus stops operating. In order to restart the image forming apparatus, it is necessary to remove the paper sheet which is the cause of the judgment that a paper jam has occurred.
Paper sheets are not always fed one at a time by the paper feeding roller. Specifically, in some cases, a plurality of, paper sheets(e.g., two sheets) may be fed at one time from the paper feeding cassette by the paper feeding roller, that is, there may occur a so-called duplicate paper feeding state.
When the paper sheets are thus simultaneously fed in a duplicate paper feeding state from the paper feeding cassette, the two paper sheets may, in some cases, be simultaneously fed to the photoreceptor through the registration rollers. Alternatively, one of the two paper sheets may, in some cases, be stopped just short of the registration rollers. That is, only the first paper sheet out of the paper sheets which are simultaneously fed may be conveyed by the registration rollers, and the second paper sheet may be stopped just short of the registration rollers. In this case, even after the first paper sheet passes through the registration rollers, the paper detecting switch continues being in an ON state because of the presence of the second paper sheet. Consequently, it is erroneously judged that a paper jam has occurred between the registration rollers, and the paper feeding roller irrespective of the fact that a paper jam does not substantially occur.
However, the second paper sheet which is the cause of the erroneous judgment that a paper jam has occurred is not particularly damaged. The paper sheet can be safely fed to the photoreceptor. Nevertheless, the image forming apparatus stops operating due to the erroneous judgment that a paper jam has occurred. In order to restart the image forming apparatus, therefore, the second paper sheet, which is not damaged must be removed. Consequently, it wastes time and labor, and paper sheets are wastefully used.